1. Field of the Invention
The invention described herein pertains to the detection of potentials existing on the surface of the skin of the living body, which potentials are generated by various sources such as muscle or nervous system activity within the body.
2. Discussion of the Background
Present biopotential detection techniques typically involve the use of conductive pastes or gels in combination with a metallic contact surface to form an electrode capable of transforming ionic current flow in the body into electronic current flow in the measuring apparatus. There are several realizations of this basic type of electrode, and all of them suffer from the various disadvantages of wet systems, such as skin irritation, loss of electrical contact due to drying paste or lead wires falling off, poor shelf life, etc. Attempts have been made to eliminate the pastes and gels in two separate ways: the introduction of new electrode materials, and the incorporation of amplifier electronics into the electrode itself.
The materials introduced in the prior art as dry electrodes generally have not been accepted in the medical field due to poor performance. For example, most of the metals and conductive composite materials such as those disclosed in U.S. Pat. Nos. 3,56,860 and 3,606,881, generate excessive low frequency electrical noise voltages when in contact with a saline solution such as the human sweat which invariably collects on the surface of the skin beneath the electrode, but there is little or no attention given to this problem in the prior art. Furthermore, many of these materials are too stiff to conform to irregularities in skin surface, thus resulting in an unstable dry skin contact subject to excessive movement artifact.
Another group of efforts to obtain disposable electrodes has sought to modify conventional non-conductive pressure-sensitive adhesives by loading them with fine conductive particles such as carbon powder (U.S. Pat. No. 3,911,906) or metal-coated plastic microspheres (U.S. Pat. No. 3,566,059). However, the conductive filler materials have tended to degrade the adhesives, requiring the electrode pad to be undesirably large if it is to adhere adequately to skin. In U.S. Pat. No. 3,911,906, for example, the preferred diameter of the adhesive pad is two inches, not to be greatly reduced, but such a large area electrode detects signals from a large volume of tissue, which is not always desirable especially if the signal of interest is from a source distant from the electrode site. In this case, the increased level of electrical activity from the muscles directly below the electrode would result in excessive interference or artifact in the recording. Second and more fundamental, conductive adhesives of this type in contact with unprepared dry skin generally do not establish sufficient electrical conduction to the body to be of particular use with present monitoring technology.
Another group of "dry" electrode materials includes a variety of conductive gums or gels which are sufficiently viscous to form the main body of a disposable electrode while simultaneously providing both its adhesion and its electrical connection to the skin. Hydrogels such as those disclosed in U.S. Pat. Nos. 3,998,215; 4,391,278; and 4,515,162 cannot truly be regarded as "dry" materials since by definition their water and/or alcohol content is typically high (for example, 30-75% by weight for U.S. Pat. No. 4,391,278). This leads to problems of electrode dryout, special packaging, and poor shelf life. A group of materials more satisfactory in this respect has been derived from polysaccharides such as gum karaya. For example, U.S. Pat. Nos. 4,125,110 and 4,229,231 describe karaya-based conductive-adhesive gels of relatively low water content for use in disposable ECG electrodes. As pointed out in U.S. Pat. No. 4,273,135, however, the variable conditions of growth and processing of these naturally occuring polymers can lead to corresponding inconsistency in the physical and chemical properties of the resulting gels. Furthermore, steps must be taken to control problems of microbial growth and skin irritation that these organic materials could otherwise engender. Consequently, synthetic polymer gels have been developed that can more consistently provide the necessary physical properties. Examples of such materials are described in U.S. Pat. Nos. 4,273,135 and 4,554,924.
Both the natural and the synthetic gels are now being used as "essentially dry" disposable electrodes (cf. U.S. Pat. No. 4,554,924 for an example). Although such electrodes represent an improvement over earlier "dry" disposables, they still suffer from limitations similar to those of the modified pressure sensitive adhesives, namely, poor skin adhesion, especially in hair and sweat, and low conductivity on unprepared dry skin. In addition, the junction between the alligator-clip lead and the foil backing of the electrode is susceptible to mechanical instability, making it a significant source of electrical interference, particularly if the lead wires are subject to movement. Finally, these gel-foil disposable electrodes remain far more costly than the reusable wet electrode technology that is still most widely used for resting or diagnostic ECG procedures. All these limitations are sharply reduced by the novel dry-electrode technology according to the present invention to be described subsequently.
Another approach to realizing a dry electrode, placing an electronic amplifier on the electrode, is based on the idea that a high impedance amplifier is able to detect a signal from a high impedance source with a minimum of signal distortion, and then drive the signal through a long cable with a minimum of interference by virtue of the low output impedance of the amplifier. One problem is that many previously disclosed designs, including DC biased transistor amplifiers, differential amplifiers, and amplifiers with gains above unity are not compatible with commonly used monitoring equipment unless some adjustment or modification is made to the monitor. Even unity-gain, DC-biased transistor amplifiers that are capacitively coupled at the output cannot conveniently be used with different types of monitors without the risk of frequency distortion caused by impedance mismatching to the different monitor inputs, and without large transient DC offsets arising when switching leads.
Another problem has been questionable reliability, as demonstrated by a group of devices utilizing a capacitively coupled input to an amplifier constructed on a metallic electrode coated with a dielectric. This type of electrode is prone to failure from dielectric breakdown due to scratches or high voltages, and exhibits undue sensitivity to external electrostatic fields.
A group of amplifiers using bipolar integrated circuit operational amplifiers with unity gain has been disclosed, but a means for adequately protecting the electronic circuitry from repeated exposure to high voltages, without compromising the essential electrical characteristics of the amplifier input, has not been demonstrated. Defibrillation voltages, static charge accumulation on the skin and clothes, and other medical equipment may cause potentials greater than 25,000 volts to contact the input to the electrode amplifier on a repeated basis, resulting in permanent failure of the device if not protected. The prior art shows the use of input resistors or unspecified current limiters for device and patient protection, but fails to show a means for compensating for the degradation of input impedance to the device as a result of parasitic capacitance coupling to ground through the resistor or current limiter. Furthermore, a single input resistor or current limiter may not provide adequate protection for some types of integrated circuit amplifiers such as CMOS devices which are sensitive to large input voltages rather than currents. Additionally, there previously has been no disclosure of a means for incorporating very small batteries into the amplifier or lead wire in order to continuously power the amplifier for a period of more than a year without prematurely exhausting the batteries. Due to excessive current demand by the amplifier, especially when the input is unconnected and the output has drifted to saturation as a result, it has previously been necessary to disconnect the batteries from the amplifier when not in use, either by physical removal or by means of a power switch, thus adding undesirable complexity to the operation of the electrode lead wire.